For a number of years there has been a growing appreciation of the danger and other disadvantages of the common practice of fixing a rotary mower blade directly on the depending drive shaft of an engine. My prior U.S. Pat. No. 3,026,665 of Mar. 27, 1962 points out that such direct drive causes engine shafts to be bent when the blade strikes an obstruction, that overloading the direct-driven blade stalls the engine, with resulting inconvenience, and that that direct drive endangers the operator and others because the blade always rotates with the engine, even during starting and idling. The seriousness of this latter danger now appears to be leading to government safety requirements for a "deadman" lever on the mower handle which will stop the blade except when the operator is in operating position and holding the lever. To be effective, a deadman control requires high reliability, and if its use is not to impose undue cost on the consumer, a highly practical, effective, and inexpensive mounting and control arrangement is required. The present invention seeks to provide such a blade mounting and control arrangement which can be applied to various presently-designed mowers with a minimum of change and expense. The invention is also useful in other applications where similar control is desirable, as on snowblowers, garden power tools, etc.